usb: renesas_usbhs: fix NULL pointer dereference in dma_release_channel()

This patch fixes an issue that the following commit causes NULL
pointer dereference in dma_release_channel().
 "usb: renesas_usbhs: add support for requesting DT DMA"
 (commit id abd2dbf6)

The usbhsf_dma_init_dt() should set fifo->{t,r}x_chan to NULL if
dma_request_slave_channel_reason() returns IS_ERR value.
Otherwise, usbhsf_dma_quit() will call dma_release_channel(), and then
NULL pointer dereference happens.
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Reported-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/renesas_usbhs/fifo.c

@@ -1072,7 +1072,11 @@ static void usbhsf_dma_init_pdev(struct usbhs_fifo *fifo)
 static void usbhsf_dma_init_dt(struct device *dev, struct usbhs_fifo *fifo)
 {
 	fifo->tx_chan = dma_request_slave_channel_reason(dev, "tx");
+	if (IS_ERR(fifo->tx_chan))
+		fifo->tx_chan = NULL;
 	fifo->rx_chan = dma_request_slave_channel_reason(dev, "rx");
+	if (IS_ERR(fifo->rx_chan))
+		fifo->rx_chan = NULL;
 }
 
 static void usbhsf_dma_init(struct usbhs_priv *priv,